Electric lamps which have a light source capsule with a generally planar seal(s) include, among others, high intensity discharge (HID) metal halide and mercury vapor lamps. The light source capsule in these lamps is a discharge vessel or arc tube of fused silica (quartz glass) which typically is sealed at both ends by a press seal which includes two major, substantially parallel faces and two minor, side faces extending between the major faces. Conductive lead-throughs extend through the press seal in a gas-tight manner to a pair of discharge electrodes arranged within the arc tube.
These lamps typically have an outer envelope which is sealed at one end by a lamp stem. A frame comprising a field wire and metallic support rods extends from the lamp stem and supports the arc tube within the outer envelope. Metallic support straps secured about the press seals are welded to a support rod on one or both sides of the press seal to secure the arc tube to the frame.
Photoelectron emission can be very detrimental to certain electric lamps. In quartz metal halide discharge lamps the arc tube contains during lamp operation an ionized plasma of mercury, sodium halide, and other metals such as scandium iodide and lithium iodide. Sodium and lithium ions have a relatively high rate of diffusion through heated quartz. Photoelectrons which collect on the outer surface of the arc tube create a negative potential that attracts the positive sodium or lithium ions and accelerates their diffusion through the wall of the arc tube. The production of photoelectrons substantially accelerates the depletion of sodium within the arc tube and thus shortens the useful life of the lamp.
In the past thirty years, many efforts have been made to reduce Na loss in quartz metal halide lamps. Nitrogen gas is normally filled in the lamp envelope to retard photoelectrons reaching the arc tube wall. Maximizing the distance between the arc tube and field wire in a lamp can reduce photoelectrons on the arc tube surface. The other method includes the use of low Na permeability quartz glass. It has been found that the use of low photoelectric emission and low thermionic emission field wire material is important to reduce Na loss. Low electrical resistivity is necessary to have low power consumption for the field wire. Sufficient high melting temperature for a field wire material is required to maintain its function over long lamp life.
U.S. Pat. No. 2,152,997 relates to mercury electric discharge lamps which have a helix of tungsten, molybdenum, nickel, and other metal welded to the inlead and extending along the envelope. The major difference in this type of lamp and metal halide lamps is that metal halide (MH) lamp contains sodium iodide and other metal iodides such as lithium iodides or bromides in the arc tube that could diffuse through the quartz arc tube. Due to the presence of metal iodides or bromides in the arc tube, a field wire should have a low photoelectric emission and low thermionic emission to minimize sodium or lithium diffusion. The field wire should be mounted in a position as far as possible to the arc tube to reduce Na or Li diffusion. These are not requirements for mercury discharge lamps because metal iodides or bromides are not present in the arc tube.
A material to be used as a field wire in metal halide discharge lamp should meet the following requirements:                electrical resistivity at 25° C.<10 ohms/mm2         photoelectric emission work function>4.0 eV        thermionic emission work function>4.0 eV        melting point>1200° C.        
Pure molybdenum wire has been widely used in the metal halide lamps for many years. Molybdenum has a high melting temperature and good electrical conductivity but it has certain drawbacks such as relatively high photoelectric emission, high cost, and less resistance to oxidation. High photoelectric emission can cause Na and Li diffusion through the quartz arc tube wall and result in poor lumen maintenance, color shift, arc tube wall blackening, and a constricted arc. In the worst case, the quartz arc tube can be devitrified and lead to a non-passive failure due to constricted arc and high wall temperature.
There is a need in the art for a field wire made of a less expensive material that also possesses the required characteristics. There is also a need in the art for a field wire that has a lower photoelectric emission and lower thermionic emission than molybdenum to reduce Na loss.